Optical transmission systems incorporating laser transmitters can be generally classified under two broad categories: analog systems and digital systems. In analog systems, the laser transmitter operates in an analog mode that can be described using the prior art optical system shown in FIG. 1. Laser transmitter 120 accepts an analog input signal via electrical line 110 and produces an output optical signal on optical link 140. In certain applications, the analog input signal may be a baseband analog signal such as a sine wave of a particular frequency, or a complex analog signal containing several frequency components. In other applications the analog input signal may be a modulated signal containing carrier frequency components. Typically, laser transmitter 120 is a broadband device that can accommodate the range of frequencies present in the input signal, and carries out an electrical-to-optical conversion process by converting the electrical-domain analog input signal into an optical-domain output optical signal.
Power controller 130 operates together with laser transmitter 120 to produce the output optical signal at a desired power level while simultaneously ensuring that laser transmitter 120 operates in a linear operative mode that produces certain desirable characteristics. Such desirable characteristics include output power accuracy, stability over time and environmental conditions, flat frequency response, and low harmonic distortion. Consequently, power controller 130 typically incorporates several temperature-stable components as well as feedback circuitry. The bandwidth and response time of the feedback circuitry is typically chosen to filter out unwanted high-frequency perturbations and to provide the desired characteristics mentioned above. As a consequence, the turn-on/turn-off times of the laser transmitter 120 is fairly long. While this may be acceptable for the analog system of FIG. 1, such lengthy turn-on/turn-off times prove unsuitable for digital mode operations wherein the laser transmitter has to be turned on and off more rapidly.
FIG. 2 illustrates a prior art optical system operating in a digital mode. Laser transmitter 220 accepts a digital input signal via electrical line 210 and produces an output optical signal on optical link 240. The digital input signal is typically a baseband signal that uses two or more logic levels. When two levels are used, the system is termed a binary system, and the laser transmitter operates in two distinct modes—either on or off. The on state corresponds to a first logic level, while the off state corresponds to a second logic level. The speed at which the laser transmitter 220 can change from one state to the other (e.g. from on to off) determines the maximum digital signal rate that can be accommodated. The degree of illumination or lack thereof in the output of the laser transmitter 220, often referred to as the extinction ratio, determines the error rate encountered in decoding the optical digital signal at a receiver located at the other end of optical link 240. For example, if the laser transmitter 220 did not turn off completely, an optical receiver at a distant end may erroneously interpret the digital logic present in optical link 240 at that instant as a logic “one” rather than a logic “zero.” Consequently, laser transmitter 220 operates in a saturated mode of operation wherein a laser inside laser transmitter 220 is completely on or completely off. This is in contrast to the analog system of FIG. 1 where a laser inside laser transmitter 120 operates in a linear manner and produces various non-discrete levels of intensities.
Power controller 230 is designed to place laser transmitter 220 in the digital mode of operation where characteristics such as extinction ratio, rise and fall times, and bit rate are the primary criteria in contrast to characteristics such as flat frequency response and low harmonic distortion that are desirable in the system of FIG. 1. It can therefore, be appreciated that the two prior art systems illustrated in FIGS. 1 and 2, prove to have certain handicaps when a mixed mode of operation (digital and analog) is desired. Thus, a heretofore unaddressed need exists in the industry to address the aforementioned and/or other deficiencies and inadequacies.